A main portion of an example of a heretofore known semiconductor device is shown in a sectional view (FIG. 9(a)) and a plan view (FIG. 9(b)).
A semiconductor device 101 shown in FIGS. 9(a) and 9(b) includes a metal base plate 102. The base plate 102 has through holes 102a for mounting the semiconductor device 101 to a heat sink (not shown) with bolts. A laminated substrate 103 is joined to the top of the base plate 102 by a joining material 104. The joining material 104 is specifically a solder. The laminated substrate 103 is formed of an insulating plate 131, a metal plate 132 provided on one surface of the insulating plate 131, and circuit boards 133 which are provided on the other surface of the insulating plate 131 and which form a predetermined circuit. The laminated substrate 103 is, for example, a DCB (Direct Copper Bond) substrate.
Semiconductor chips 105, such as IGBTs (insulated gate bipolar transistors), are electrically and mechanically connected to the circuit board 133 by respective conductive joining materials 106. “Being electrically and mechanically connected” is defined as including not only a case in which objects are connected together by direct joining, but also a case in which objects are connected together via a conductive joining material, such as a solder or a metal sintered material, and the same applies in the following description.
As the base plate 102 and laminated substrate 103 of the heretofore described of semiconductor device 101 are different in heat expansion coefficient from one another, heat stress is applied to the joining material 104 due to the heat generation when the semiconductor chips 105 are in operation. Further, when the semiconductor device 101 is in operation, there is fear that cracks are generated in the joining material 104 by repetition of the heat stress. FIG. 10 shows an ultrasonic observation image, wherein the condition of cracks generated in the joining material is observed. FIG. 10 is an image of the joining material 104 seen from above, wherein the white portions show cracks. As shown in FIG. 10, cracks are generated in an outer periphery, such as at four corners, of the joining material 104, and grow inward. Also, in order to prevent the generation and growth of cracks in the joining material 104, it is effective to form the joining material 104 to a certain thickness.
In order to form the joining material 104 to a certain thickness, projections 121 are disposed on the principal surface of the base plate 102. The distance between the base plate 102 and the metal plate 132 is defined by the projections 121, thus obtaining the joining material 104 with the same thickness as the height of the projections 121.
However, it is not possible to sufficiently secure the thickness of the joining material 104 in the outer peripheral portion of the metal plate 132 in the case of the semiconductor device 101 including a laminated substrate 103A, a convex warp of which to the circuit board 133 side is large, as shown in FIG. 11. Therefore, there is fear that the cracks are generated in the outer peripheral portion of the joining material 104. The warp of the laminated substrate 103A occurs due to, for example, the difference between the total volume of the circuit board 133 and the total volume of the metal plate 132.
Even in the case of the laminated substrate 103A, where the convex warp at the circuit board 133 side is large, it is possible to prevent the generation of cracks by increasing the height of the projections 121 and increasing the thickness of the outer peripheral portion of the joining material 104. However, the increase in the height of the projections 121 also leads to an increase in the thickness of the central portion of the joining material 104 which is not high in heat conductivity. Therefore, there is fear that the heat conductivity from the metal plate 132 to the base plate 102 deteriorates, and that the heat dissipation property from the semiconductor chips 105 to the external deteriorates.
PTL 1 describes a semiconductor device wherein a projection on a base plate is positioned in a region separated 1 to 10 mm from the peripheral edge of a circuit board of a laminated substrate, and a groove of a rectangular shape in section is provided on the side closer to the peripheral edge than the projection. Even in the case of a laminated substrate whose warp is large, the groove serves to maintain the thickness of a joining material between the base plate and the circuit board of the laminated substrate. However, the groove of the base plate described in PTL1 has a space of a rectangular shape in section. As the region in the groove in which the joining material is thick is wide when the groove is rectangular, there is a room for improvement in the heat dissipation property from the laminated substrate to the base plate. Also, the groove described in PTL 1 is such that it is difficult for voids in the joining material to come out when joining, and that there is a room for improvement in the reliability of the joining material.
Also, PTL 2 describes a semiconductor device wherein grooves are formed in a surface of a base plate in order to bring the base plate into close contact with a heat sink, thereby controlling the warped shape of the base plate. However, the grooves described in PTL 2 do not contribute effectively to an adjustment of the thickness of a joining material.